Polyether polyols are integral intermediate components utilized to manufacture a wide array of products, including polyurethanes. As such, the production of polyether polyols is critical. It is known in the art that polyether polyols are produced from the polymerization of epoxides, such as ethylene oxide (EO) and propylene oxide (PO). It is also known in the art that double metal cyanide (DMC) catalysts are effective catalysts for the polymerization of the epoxides. DMC catalysts produce polyether polyols having narrow molecular weight distributions as well as relatively low unsaturation.
In conventional methods, DMC catalysts are synthesized by combining an aqueous solution of a metal salt and an aqueous solution of a complex metal cyanide salt. As a specific example, an aqueous solution of ZnCl2 (excess), as the metal salt, is combined with an aqueous solution of K3Co(CN)6, as the complex metal cyanide salt. This combination precipitates out the desired DMC catalyst, in this case specifically Zn3[Co(CN)6]2. Examples of such conventional methods are disclosed in U.S. Pat. Nos. 5,470,813 and 5,714,639. These conventional methods, in one form or another, utilize a complex metal cyanide salt. The complex metal cyanide salts are very expensive which limits the economic viability of utilizing DMC catalysts in the production of polyether polyols.
In other conventional methods, DMC catalysts are synthesized by combining an aqueous solution of a first metal salt with an aqueous solution of a second metal salt and with an aqueous solution of an alkali metal cyanide in a single step to synthesize the DMC catalyst. Examples of such conventional methods are disclosed in U.S. Pat. Nos. 6,436,867 and 6,593,268. Although such methods do not rely on use of a complex metal cyanide salt, they remain deficient because these methods do not utilize sonication during synthesis of the DMC catalysts. Without making use of sonication, these methods synthesize DMC catalysts with increased particle size and, thus, with less active surface area per particle of DMC catalyst. Furthermore, the DMC catalysts synthesize according to these methods without sonication during synthesis have less desirable morphology with inconsistent surface uniformity which makes active sites of the DMC catalysts less accessible when used to produce a polyether polyol.
U.S. Pat. No. 6,596,842 to Eleveld et al. discloses use of sound waves, including ultrasonic sound waves, to ‘treat’ a DMC catalyst that is used in a process for polymerizing alkylene oxides to produce polyether polyols. Typically, the '842 patent treats the DMC catalyst while the DMC catalyst is being transported to the reactor that is used to produce the polyether polyols. The '842 patent also notes, however, that the DMC catalyst can be treated in a catalyst slurry. This catalyst slurry is generated after that DMC catalyst of the '842 patent has already been synthesized and it is merely a mixture of a small amount of the DMC catalyst and a polyether polyol. In short, the '842 patent does not utilize any sound waves for treatment during actual synthesis of the DMC catalyst. As such, the DMC catalyst of the '842 patent, which is to be subsequently treated with sound waves, is already deficient synthesis of the DMC catalyst itself. Furthermore, the DMC catalysts of the '842 patent are synthesized using complex metal cyanide salts, and these catalysts, as described above, are not economically attractive.
Due to the deficiencies of conventional methods for synthesizing DMC catalysts, including those described above, it would be desirable to provide a method of synthesizing DMC catalysts that does not utilize expensive complex metal cyanide salts as intermediates thereby improving the economic viability of DMC catalysts utilized in the production of polyether polyols. It would also be desirable if the method provides a DMC catalyst that has decreased particle size with more active surface area per particle of the DMC catalyst as well as improved overall morphology.